This invention relates to yield mapping, and is particularly concerned with improving the accuracy of yield maps.
In recent years it has become customary to monitor a crop""s yield during the course of harvesting. In a combine harvester this is often done using a yield monitor, which measures the amount of crop material harvested. There are many types of yield monitors available. One such monitor is described in International Patent application No. 96/38714.
Over the years there has been a tendency for field sizes to increase. Formerly there would have been little variation in yield in any one given field, and as such the farmer could plan his inputs so that they were uniform across the field. However, field sizes have increased by eliminating the boundaries between fields and creating one large field where three or four smaller fields had existed before. The result is that in fields presently farmed, there are often large variations in yield capability across the field.
The availability of reasonably accurate yield monitors, and the need to treat different areas of the same field differently led to the farm management practice known as xe2x80x9cprecision or site-specific farmingxe2x80x9d, whereby the farmer tailors inputs to requirements.
Precision farming often involves the farmer in equipping his harvesting machine (such as a combine harvester) with a yield monitor and a positioning system so that the crop yield at any given position can be established. The farmer can then use this information to establish a yield map, and in turn an application map. Inputs are then applied to the crop at varying rates according to the application map.
The philosophy behind precision farming is to reduce the total cost of inputs by selecting the amount of inputs according to need. Since inputs are often based on yield maps it is important for the maps to be accurate. It has been noted that there are several possibilities for error in yield monitoring when harvesting a crop with a harvesting machine.
Existing yield meters used on-board combine harvesters have a number of problems, which may lead to errors in yield maps. Existing yield meters function as set out below:
a. Data is only recorded when the machine is harvesting, i.e., the threshing mechanism is engaged, the table is engaged, the combine is travelling more than 1 km/h and the table is below 0.5 m.
b. It is assumed that only valid data is recorded when the combine is in harvest mode.
c. Simple filtering is used to delete large errors in the raw data set.
d. Any remaining errors in the raw data are smoothed during the interpolation process by amalgamating them with accurate data. The interpolation process hides errors in the data set.
Seven different types of error have been identified when monitoring yield and position on a combine harvester, and these errors are listed below:
1. Measured areaxe2x80x94the measured area is calculated by multiplying the cutter bar width by the distance travelled by the combine wheels. The width of the cutter bar and the rolling circumference of the wheel are entered into the combine""s computer by the operator. As these values are not monitored during harvesting, they are assumed to be constants until manually adjusted. If they deviate from the programmed values, then errors are introduced into the raw yield data set when converting grain flow (kg/s), measured at the yield meter, into grain yield (t/ha).
2. Delay for yieldxe2x80x94this is the true position related to the measured grain flow. This represents the time delay from the crop being cut (true position), and travelling through the combine to the point where the grain flow is measured by the yield meter.
3. Lead timexe2x80x94there is a period of time that is required for the combine to fill with grain once it has entered the crop. This lead time is, therefore, the time lapse that is required for the combine to reach a stage when it has filled with grain and is giving a true reading on the yield meter. From this point reliable yield data can be logged.
4. Lag timexe2x80x94this is time required by the combine to empty of grain once it has stopped cutting the crop at the cutter bar. The lag time therefore, represents the time that true yield data can be logged once the combine""s cutter bar has left the standing crop.
5. Delayed positioningxe2x80x94this is the offset of the GPS antenna. For practical reasons it is not possible to mount the GPS antenna directly over the cutter bar and it is normally mounted on the cab roof of the combine harvester. The location of the GPS antenna on the cab roof introduces a xe2x80x9cpositioning offsetxe2x80x9d.
6. Turing on headlands without cutting any cropxe2x80x94errors are recorded in raw data when the combine is turning on the headland with no grain being cut. In this instance 0 t/ha (zero) will be recorded in the data string which will influence the processed yield map.
7. Tramlinesxe2x80x94although not an actual error, because of any reduced yield resulting from wheelings, tramlines could have potential to cause a stripping effect in a resulting yield map.